linux-kernel - Re: [patch 00/11] ANNOUNCE: "Syslets", generic asynchronous system call support

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Date:	Tue, 13 Feb 2007 23:50:56 +0100
From:	Olivier Galibert <galibert@...ox.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Davide Libenzi <davidel@...ilserver.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Christoph Hellwig <hch@...radead.org>,
	Andrew Morton <akpm@....com.au>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Ulrich Drepper <drepper@...hat.com>,
	Zach Brown <zach.brown@...cle.com>,
	Evgeniy Polyakov <johnpol@....mipt.ru>,
	"David S. Miller" <davem@...emloft.net>,
	Benjamin LaHaise <bcrl@...ck.org>,
	Suparna Bhattacharya <suparna@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [patch 00/11] ANNOUNCE: "Syslets", generic asynchronous system call support

On Tue, Feb 13, 2007 at 10:57:24PM +0100, Ingo Molnar wrote:
> 
> * Davide Libenzi <davidel@...ilserver.org> wrote:
> 
> > > Open issues:
> 
> > If this is going to be a generic AIO subsystem:
> > 
> > - Cancellation of pending request
> 
> How about implementing aio_cancel() as a NOP. Can anyone prove that the 
> kernel didnt actually attempt to cancel that IO? [but unfortunately 
> failed at doing so, because the platters were being written already.]
> 
> really, what's the point behind aio_cancel()?

Lemme give you a real-world scenario: Question Answering in a Dialog
System.  Your locked-in-memory index ranks documents in a several
million files corpus depending of the chances they have to have what
you're looking for.  You have a tenth of a second to read as many of
them as you can, and each seek is 5ms.  So you aio-read them,
requesting them in order of ranking up to 200 or so, and see what you
have at the 0.1s deadline.  If you're lucky, a combination of cache
(especially if you stat() the whole dir tree on a regular basis to
keep the metadata fresh in cache) and of good io reorganisation by the
scheduler will allow you to get a good number of them and do the
information extraction, scoring and clustering of answers, which is
pure CPU at that point.  You *have* to cancel the remaining i/o
because you do not want the disk saturated when the next request
comes, especially if it's 10ms later because the dialog manager found
out it needed a complementary request.

Incidentally, that's something I'm currently implementing for work,
making these aio discussions more interesting that usual :-)

  OG.
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